Hi John,
I understand now, it's k_x, not k*x. This is just the phase constant (β) to which I was referring - so what you need to do is take my equation Angle = β*d, and get β = Angle/d.
This is the tricky part - it doesn't give units for k. It could be rad/m or degrees/m. It appears to be radians, from the graph title, so you would sweep from 0 degrees to
Angle = 1800*(pi/180)*(d)
However, to be thorough, you should really just sweep 0 - 180 degrees. This might be the same case if your unit cell is the proper size, however.